UXP/layout/reftests/css-valid/button/style.css

/* Override default style */
button:invalid {
  box-shadow: none;
}

button.notvalid {
  background-color: green;
}
button.notvalid:valid {
  background-color: red;
}

button.valid {
  background-color: red;
}
button.valid:valid {
  background-color: green;
}